1. Field of the Invention
Embodiments described herein relate to the field of seismic exploration. More particularly, to an apparatus and method of transferring seismic equipment to and from an operations platform and an underwater location.
2. Description of the Related Art
Seismic exploration operations in marine environments typically are conducted from the deck of one or more seismic exploration vessels, such as floating platforms or ships. While the fundamental process for detection and recording of seismic reflections is the same on land and in marine environments, marine environments present unique problems due to the body of water overlaying the earth's surface, not the least of which is moving personnel and equipment to a site and maintaining them there for an extended period of time. In this same vein, even simple deployment and retrieval of seismic receiver units in marine environments can be complicated since operations must be conducted from the deck of a seismic exploration vessel where external elements such as wave action, weather, and limited space can greatly affect the operation.
These factors have become even more significant as exploration operations have moved to deeper and deeper water in recent years, where operations require longer periods of offshore time. Seismic exploration in deep water typically uses seismic receiver units that are placed on or near the seabed. These devices are typically referred to as Ocean Bottom Cabling (OBC) or Ocean Bottom Seismometer (OBS) systems, such as Seafloor Seismic Recorders (SSR's). These SSR devices contain seismic sensors and electronics in sealed packages, and record seismic data on-board the units while deployed on the seafloor as opposed to digitizing and transmitting the data to an external recorder. The recorded data is retrieved by retrieving the units from the seafloor. SSR's are typically re-usable and may be recharged and serviced before re-deployment.
In a typical operation, hundreds or thousands of OBS units are deployed in a seismic survey. In one conventional method, the OBS units are deployed using a remotely operated vehicle (ROV) tethered to the operations platform. The ROV may be pre-loaded with OBS units on an on-board storage compartment, and the ROV is lowered below the surface of the water. The ROV is then positioned subsurface and one or more OBS units are removed from the storage compartment and placed on the seafloor. Once all OBS units are removed from the storage compartment, the ROV must be brought to the surface and/or the operations platform for reloading of OBS units. Once reloaded, the ROV is again lowered and the OBS subsurface placement operation is resumed. Likewise, retrieval of OBS units requires transfer of the OBS units from the seafloor to the ROV and, once the storage compartment is full, the ROV must be brought to the surface and/or operations platform for unloading of the OBS units to the operations platform.
The conventional ROV's are typically heavy and require a sophisticated tethering arrangement to control various functions on the ROV. As such, larger vessel cranes operating at low lifting/lowering rates are required for ROV handling. Thus, lifting and lowering the ROV takes time and the lift/lowering time increases proportionally with the depth of the water. Also, heavy seas and/or currents may increase the possibility of damage to the ROV and/or tether each time the ROV is lifted or lowered. Further, potential personnel safety issues are of a concern each time the ROV is lifted and lowered.
Therefore, what is needed is a method and apparatus for transferring OBS units to and from the ROV after the ROV has been deployed in order to minimize the frequency of lifting and lowering the ROV.